The invention pertains to an oscillatory drive unit for use in an oscillating system to be oscillated substantially at the resonance point of the oscillating system. The oscillating system generally includes a mass to be oscillated which mass is resiliently suspended on or supported by a countermass by means of any kind of resilient springs. Although the invention has particular utility in connection with oscillating conveyors that are to be operated substantially at their resonance point, the problems to be solved with the object of the invention mainly occur also in other oscillating systems that primarily operate within the resonant range.
In prior art oscillating conveyors, the resiliently supported oscillating or conveying bodies are subjected to a forced oscillation by means of a vibrator. This oscillation causes the material situated on the oscillating conveyor to travel along the conveyor. There exist so-called oscillating conveyors that are subjected to an essentially linear oscillation, the direction of which approximately corresponds to the initial angle of a parabola. Such oscillating conveyors are frequently supported in resilient fashion on leaf springs that are arranged perpendicular to the desired oscillating direction and allow a practically linear oscillation in this direction. This practically linear oscillation only deviates from linearity due to the circular arc path of the spring ends. This deviation may be neglected if the difference between the oscillation stroke and the length of the spring is sufficiently large.
In addition to electromagnetic vibrators, it is also possible to utilize piston vibrators that are pressurized with compressed air as oscillatory drives for such oscillating conveyors. The latter provide the advantage that their oscillating behavior can be better adapted to different requirements.
With respect to the expenditure of energy, it is most favorable to operate oscillating conveyors within the resonant range. Within this range, the size of the masses to be moved is inconsequential. It simply must be ensured that the energy consumed by the damping in the springs and by the moved material is replaced. In this case, only part of the weight of the material to be conveyed consumes energy. Resonance conveyors are preferably realized in the form of conveyor chutes. With respect to the operating requirements, it is, however, disadvantageous that the resonance frequency of a system depends on the respective load of the material to be conveyed and varies accordingly. In known systems, the adaptation of a resonant drive to the resonance frequency of a system represents an almost insurmountable problem. An adaptive variation of the excitation frequency during the operation of such a conveyor can only be realized with significant expenditures. Consequently, measures of this type are frequently relinquished, and the oscillating resonance drives are designed in such a way that the operating range is displaced into the ascending part of the resonance curve, i.e., far away from the resonance apex. In this case, at least part of the resonance amplification can be utilized, and a damping of the amplitude while the system is subjected to a load may, for example, be compensated due to the fact that the resonance point is simultaneously displaced to a slightly lower frequency and the fixed operating frequency is displaced into a region of higher amplitude.
Even when using a more flexible piston vibrator, the adaptation of its frequency to the resonance frequency of a system is only possible under certain conditions. In piston vibrators, the power essentially depends on the piston diameter. However, the frequency and the oscillation amplitude result from the piston weight. If a heavier piston of the same diameter is used, the oscillation amplitude is correspondingly increased and the frequency is correspondingly decreased. This applies to cylindrical as well as stepped or differential pressure piston vibrators. In order to alter the oscillating behavior of a piston vibrator, pistons with different lengths and identical diameters are frequently utilized so as to attain different moments within the same power range. Normal piston vibrators that are coupled to a system to be subjected to oscillations usually comprise a piston without a piston rod which only moves back and forward within the vibrator housing. However, it is also possible to provide one side of the piston with a piston rod that extends out of the housing. The one-sided effect of this piston rod, which reduces the piston surface, makes it necessary to realize this vibrator in the form of a differential pressure piston vibrator. The piston rod that extends out of the housing may be additionally provided with masses that reduce the frequency. An adaptation of the frequency of a piston vibrator to the resonance frequency of a system would only be possible by lowering the air pressure. However, the power of the piston vibrator varies exponentially with the frequency, i.e., this measure can generally be precluded. In conventional systems, the springs of a resonance conveyor are usually designed in such a way that the resonance frequency of the system lies within the economical operating range of a certain vibrator type. This may, for example, be realized by altering the piston mass.
Resonance conveyors are not only sensitive to load changes. The natural frequency essentially also depends on the ratings of the springs used and, for example, the weight of the conveyor. If the conveyor is structurally modified such that its weight is changed, it is possible that the conveyor no longer resonates and consequently conveys no material. In conventional so-called oscillating resonance conveyors that operate at a frequency of only approximately 85% of the resonance frequency, it is a customary procedure to operate below this frequency in order to still attain a noticeable resonance amplification of the oscillation amplitude and reduce the sensitivity of the system. If a conveyor that is correctly calculated and designed with respect to its resonance frequency is assembled on a foundation or frame that is insulated against oscillations, i.e., assembled elastically, so as to transmit the least possible oscillations onto the substructure, the mass of the foundation and its elastic support must be incorporated into the calculation, i.e., a system, the drive of which is generally adapted to the resonance frequency, no longer resonates.
The present invention is based on the objective of developing an oscillatory drive for a system with an oscillating mass and, in particular, an oscillating conveyor, in which the oscillatory drive practically recognizes the respective resonance frequency of the driven system, follows changes of this resonance frequency, and supplies the system with the energy required for maintaining the oscillations without electrical recognition or control units.
According to the invention, this objective is, in principle, attained with the characteristics disclosed in the characterizing portion of claim 1.
Although a person skilled in the art will easily ascertain that the invention can also be utilized in other oscillating systems, e.g., vibration tables, screens, filter frames, etc., the invention is described below with reference to a resonance conveyor.
Leaving aside the corresponding coupling of the individual elements, it is an essential aspect of the invention to provide an oscillatory drive, the mutually moving components of which are not positively limited with respect to their mutual oscillation amplitude, e.g., by means of a limit stop, but rather able to essentially freely adapt to a resonance oscillation of the system. In this case, it is not precluded that a certain (progressive) elastic limitation for the mutually oscillating parts is provided. Consequently, the times at which the drive supplies energy are not dependent on the reversal points of a drive component, e.g., a piston. On the contrary, the energy may be supplied at an interim phase during the acceleration cycle of a moving drive component in order to excite this drive component to carry out amplified oscillations. It was determined that the system also operates flawlessly if the energy is supplied slightly before the reversal point.
According to the invention, the pistons of these oscillatory drive units must be able to follow the resonance oscillation of the system without impacting with a rigid limitation in the cylinder. The supply of compressed air into the cylinder chamber behind the trailing end of the piston is, for example, controlled out by the piston, e.g., during the acceleration phase of the piston after an inlet opening is released.
In order to make it possible to transmit the energy that is introduced into the oscillatory drive and must be converted into an oscillation movement as directly as possible onto the oscillating system, the coupling of the oscillatory drive to both masses in question should be as rigid as possible in the direction of the transmission of oscillations. However, if the shape and direction of the oscillations of the drive do not exactly correspond to the possible oscillation path of the driven system, it is preferred to utilize flexible or articulated rods for the coupling. These flexible or articulated rods are relatively rigid in their longitudinal direction, but allow a lateral excursion during the oscillation sequence, i.e., the piston in the oscillatory drive is not unnecessarily stressed by transverse forces.
A certain elasticity of the coupling elements in the direction of the transmission of oscillation would also be conceivable, e.g., in order to ensure that a certain reserve for the oscillation amplitude is still available at high oscillation amplitudes and an unexpected impact of the oscillatory drive piston in the housing. However, this solution is associated with the disadvantage that the oscillation energy is transmitted to the system with certain delays because such an elastic coupling of the piston creates a separate oscillating system that tends to oscillate in resonance and consequently can make it impossible to control the sequence of the energy transmission. In any case, it would be better to design the oscillatory drive in such a way that the possible piston travel suffices for the oscillation amplitudes of the system which occur during its operation.
The countermass, on which the oscillatory drives of such conveyors are supported, can consist of a stationary foundation. In this case, the countermass is rigid or practically rigid and has an essentially infinite size, i.e., the resonance frequency of the conveyor is primarily determined by its own mass and the type of its resilient support. However, the countermass may also consist of a pedestal frame of the device that is elastically supported on a foundation or substructure so as to transmit the fewest possible oscillations onto the substructure. In contrast to a rigid support, such an arrangement alters the resonance frequency of the system. According to the invention, the countermass may also consist of a mass that is freely arranged in space and held by springs that connect the countermass to the conveyor. In this case, it is necessary to suspend or support the entire system. This may, for example, be realized by suspending the entire system on the conveyor by means of very soft springs that limit the degrees of freedom of the entire system as little as possible. A system that is suspended on soft springs usually has a different, essentially lower resonance frequency than that of the driven, insulated system consisting of both masses and their mutual spring coupling. The main disadvantage of such an arrangement can be seen in the fact that the oscillation amplitude of the conveyor referred to its surroundings is significantly reduced due to the lack of a support for the countermass.
Oscillating conveyors should generally cause a movement of the material to be conveyed during each oscillation, namely such that their oscillations are directed transversely upward viewed in the conveying direction of the material. A degree of elastic freedom in such a direction and the most rigid arrangement possible in the other directions is preferably realized with leaf springs that extend perpendicular to the intended oscillating direction. The ends of such leaf springs do not have a strictly linear path. As long as their oscillation amplitude is small in relation to their size, the circular arc movement can be considered to be practically linear.
It is not absolutely imperative that the oscillating direction of the oscillatory drive corresponds to the direction of the degree of oscillating freedom of the conveyor. However, the conveyor is usually designed in this fashion because only a certain component of the moment of the oscillatory drive would otherwise be used for generating the oscillations.
If only one oscillatory drive is provided in a conveyor, the elastic coupling of which to the countermass does not inevitably define the oscillating direction, it is required that the line of application of the oscillatory drive extend through the center of gravity of the oscillating mass of the conveyor. If the countermass is elastically supported, the direction of the line of application of the oscillatory drive also must extend through its center of gravity. This can be attained by designing the shape of the mass correspondingly. If the conveyor is suspended on leaf springs, it is also practical to observe this rule. However, the arrangement is less critical if several oscillatory drives are provided. Since the systems in question pertain to systems that oscillate in resonance, only one correspondingly designed oscillatory drive suffices in most instances.
The conventional crank drive also represents an oscillatory drive that is supported on a foundation frame with one end and engages on an elastically supported mass with the other end. However, the frequency and amplitude of a crank drive are predetermined during its operation, i.e., only one forced oscillation can be generated with a crank drive. As mentioned previously, it is essential for the invention that an oscillatory drive, in which the position of the reversible points is not defined, is utilized so as to ensure that the piston of the oscillatory drive which is connected to the system is able to follow the oscillations of the system in largely unobstructed fashion, i.e., its oscillation amplitude as well as its oscillation frequency are essentially also defined by the system. Due to this measure, the oscillatory drive assumes or recognizes the respective resonance frequency of the entire system.
According to the invention a pneumatic oscillatory drive unit is provided that is pressurized with the pressure medium on only one side, i.e., only from one of its reversible points, and subsequently ventilated. This piston vibrator is able to freely oscillate in the direction towards its other reversal point, with the return movement being realized without the assistance of external energy, namely by the coupled system that oscillates backward. In piston vibrators that are provided with a piston rod that extends out of the housing and charged with pressure on both sides, it is possible to operate exclusively with a differential pressure. However, in a vibrator that is pressurized with pressure on only one side, preferably the side situated opposite to the piston rod, the entire piston surface is available for pressurizing the piston. This is particularly advantageous for the start-up of a system that operates in resonance. Before the resonance frequency is reached during the start-up, higher restoring forces of the springs usually must be overcome. If it is ensured that a vibrator that is pressurized on one side is situated in a suitable start-up position in the idle state of the device, the system can be started without problems by pressurizing the vibrator with the full pressure. Since the system automatically transforms into the resonance mode during its operation, a change in the pressure of the operating medium and consequently the supplied energy only causes the oscillating amplitude to change. The required supplementary energy supplied to the system can be adapted solely by changing the pressure.